Conventionally, a heater core using an exhaust heat of an engine is well-known. According to Japanese Patent No. 3278919, when a heating is started in a case where a temperature of a coolant of the engine is low, a cold air flows to passengers. Therefore, when the temperature of the coolant is less than a predetermined temperature, the heating is not started.
However, in a vehicle provided with an exhaust heater and an electric heater which are used as heating devices, the optimum start timing of the heating cannot be determined only by the temperature of the coolant.